Why Do Capuchin Monkeys Cover Themselves In Smelly Substances?

A capuchin monkey. Scott Walmsley/Shutterstock
Robin Andrews 14 Oct 2015, 15:08

Capuchin monkeys are smart little critters, to say the least. To get to their favorite nuts, they use tools made of quartz, limestone and sandstone to crush them open against wooden surfaces, changing to a stronger material if the nut is particularly resilient. Despite the fact that they clearly have high levels of analytical and social intelligence, a new study in Scientific Reports reveals that they have something in common with many less intelligent creatures: capuchin monkeys will anoint themselves with a range of pungent substances made from food and vegetables.

You may not be aware, but the idea of a wild animal scenting up before attending a social event isn’t actually new. Lemurs, deer, elk, goats, rats, bears and even the docile European hedgehog often spread an odoriferous substance on their bodies for a whole variety of reasons. The hedgehog’s unguent of choice – comprised of a delightful mixture of toad skin, tobacco, soap and feces – is used as a form of scent-based camouflage, or perhaps as a method of toxifying its defensive spines. The North American brown bear makes a paste of a local root, mixes it with its saliva, and uses it as a soothing cream to stop insect bites itching.

Anointing in the non-primates of this group appears to be primarily for medicinal purposes, although scent camouflage and aiding in sexual selection have also been put forward as possible motivations.

Capuchin monkeys, it seems, may not be so different to this eclectic collection of self-anointing animals. Using various mixtures of squashed millipedes, ants, limes and onions, they will anoint themselves both in private and in the presence of others. Younger monkeys often observe the elder, dominant members of their societies to learn a skill visually before attempting to execute it themselves, and anointing appears to be no exception.

Additionally, previous studies observing both wild and captive capuchin monkeys noted an increase in aggression when mainly onion was used, they suggest that this is a response of more dominant males, who are essentially berating the onion-dousers for using resources that could also be used as food. When millipedes were the primary ingredient, there was no evidence of social competition, and even monkeys that would normally avoid each other came together to anoint, implying that there is a strong social aspect to this slightly unusual behavior.

To investigate the motive behind these behaviors, the researchers observed captive capuchin monkeys deal with various stocks of anointment material. In some cases, there were plenty of materials available to make ointment; in other cases, resources were rare.

Regardless of the level of stock, the capuchin monkeys engaged in social anointing, sharing out the resources fairly evenly; this clearly shows that anointing has a purpose beyond that of merely gaining access to resources. In fact, capuchin monkeys grooming themselves used anointing to reach hard-to-see body parts that were more difficult to groom, whereas social anointing targeted body parts that often require other monkeys to reach. Furthermore, the use of an onion did not, unlike previous studies, provoke any aggressive responses.

The researchers suggest that, instead of a particularly mysterious social urge driving the anointment procedure, it is simply to apply repellent substances to areas that have been or are likely to be affected by parasites. The entire body of each capuchin was covered in the ointment, the ingredients of which contain chemical compounds that repel insects and parasites.

For now, it appears capuchin monkeys don’t “scent up” to try and woo a mate. This is in direct contrast to spider monkeys, which use a combination of plant species to do just that.

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